Corrosion resisting steels



Patented July 8, 1952 2,602,737 a *coitnosioNnEsIsTmG STEELSY I I1 '1 ":WilliamQiBinder and James ThompsomNia gara K Falls, N..Y., assignors to Union Carbide and CarbonCorporation, a corporation of New York; NoDi-awlng. Application May Serial No. 92,4 9?

2 Claims. (ems-'12s) I v 1 This invention relates nickel steels for use in chemical service in, the welded condition, and more particularly to a method for enhancing; the' resistance of such steels to intergranular corrosion: ,1 v

Austenitic chromium-tnickelsteels, for example the.18% chromium-8% nickel and 25% chromium-20% nickel are noted for their excellent resistance to corrosion However, the precipitation of chromium carbide, which occurs when the steels are heated at temperatures between 400 and 900 C. suchsas during-welding, renders thesesteels susceptible to grain boundary. attack. In this condition, the. steels may failfby intergranular corrosion when exposed tojrelatively weak corrosives. so

A number of methods to eliminate thesintergranular susceptibility of these steels have been proposed; namely, heat-treating at temperatures above about 1000" C. andcooling rapidly, lowering T .of carbon content, cold subsequently precipitating-the carbides and rendering them harmless byfsuitable coagulating 'heat-treatments, adjusting the chromium or nickel contents to form an austenitic structure with some delta ferrite,and introducing such elements as columbium ortitanium, whichpreferentially combine with carbon and form more insoluble carbides than chromiumr- I None of these methods has proved completely satisfactory by reason of such factors as cost, limited practicality, inadeciuate'corrosion resistance, decreased hot-workability, limited weldability and formability, and combinations of these and related factors. For example, to reduce costs there is a great deinandgior cheaper au'stenitic chromium-nickel steels; than. those containing columbium or titanium having, good weldability and good resistance to inter'granular attack after limited periods of heating i-nthe carbide precipitation temperature range yin the majority of service conditions, in which the austenitic chromium-nickel.steelsareusable, the steels'are not exposed to the: detrimental temperature range iorprolonged periods oi 'ti ne, henceqthey do xnot to austenitic chromiumlow-carbon steels because of carbon pickup pduring melting and processing which renders :them

exposed for only limited periods :of timeattem working the steel and peratures in the carbide: precipitation range. The invention; by" means ofwhichithese objectives are achieved, is based on the discovery that the resistance of austenitic chromium-nickel steels, containing 16% to 25% chromium, 5% to 25% nickel, 11pm 3% molybdenum, up to 0.2%

nitrogen, and up to about;0.:06% carbon, to damaging carbideprecipitation after short periods of heating in the carbide precipitation" temperature range is-substantially increased if the composition of the steel is so 'regulated-thatit falls onrthe phase boundary lineseparating fully austenitic steels from steels containing some ferrite; -The invention accordingly comprises "such steels to need the,completa-immunitym sensitizationto intergranular corrosionthat-more extendedheat ing requires; lt haspeen proposed JIQ use steelscontaining 0.03 5% naximum carbon; for this purpose because they possess gexcellent' resistance to intergranular attack after periods of vex;-v posure, totemperaturesin the sensitizin range,

. u i o e .enc ptems in .wels naend V teels, it, afield-. 419; ma ---the e x rem l e ar ch a r: ti e he' ul vstabi ize which has been imparted'resistance 'topintergranular corrosion after exposure to conventional fabricating heating cycles such as welding and stress-relieving, for example,=by*balancing the composition ofsuch steels so thatthey are-ful1y austenitic in the annealed-condition-by-incorporating nickel in such steels in aproportion determined bythe proportions of thezother iconstituentsand an equation to be given: below. While itisgenerally felt that austenitic; structure is advisable fromi the T- standpoint of"; hotworkability and corrosionfresistance,{ susceptibilityto intergranu-lar attack.:.increases. :as the alloyvcontent Of 'thG- QGBI iSZ increased'beyond that required; by the phase boundar lie tationshiptto form a c l t aust mti structurel V In general", the methodoflthe invention is applicable tosteels c0ntaining:16%ft0 25%,,ch-rom'ium, 5% to 25% nickel; and-at :1east'150% fir'on. I

Such steels conyenti onallynontainaup to =1-%"si1i- -2 t 1 ma sanese;.butit is-ipreferred that the; silicon and; man anese contents :Bmie y; i

--'the damaging range are'fshort. It

"rating molybdenum the stee Fd t i i pose; the molybdenum content should exceed should not exceed 0.04%. Nitrogen may be present up to 0.2%, and it should not be less than 0.10%. Such steels may also contain up to 3% molybdenum.

In regulating the composition of the steel for the purpose of obtaining enhanced resistance to intergranular corrosion, as described herein,it is convenient to have the phase boundary between fully austenitic steels and steels composed of austenite and ferrite expressed mathematically may be found in the literaturea'nd in general;

they reveal the attendant micro-structure for a given composition. These equations show they extent that variations in carbon, nitrogen, man;-,; ganese, nickel, chromium, silicon, and molybde num affect the structure balance of a steel. However, it is well recognized that these relationships represent the conditions under which the steels were tested and. they do not hold if the conditions differ appreciably from those employed in the development of the equation. For example, an equation developed for wrought metal cannot be applied directly'to cast metal or welds because of the differences'in homogeniety which exist between the structures and such difierences change the value of the'co'nstant term inthe equation. Another factor influencing the constant term is cold-work. An equation, based on the amount of ferrite present after a given'amount of cold-work, will difier significantly in. the constant term from one'derived on the basis of the amount of ferrite present after annealing onlyi For these reasons, it is preferred to use the following equation because it is based on the evidence supporting the inventionand therefore is'more suitable in carrying out the method described herein:

-30 c.%o 1'se %N).-0.5 (%'Mn) The" equat ion can be used to determine whether a 'steel'is ruuy austenitie' orpartia llyferritic in the' anneaIedco'nditionf without examining its micro-structure. l o'r'example, if the nickel content of the steel is greatei'than that given by the equation, the steel will be'fully austenitic, and if less, it willbe partially ferritic".

A feature of the invention is the anneal proportioning of the nickel and nitrogen contents. If the nickel content is significantly greater than the amount required to render the steel in' the annealed condition completely austenitic in accordance with the phase boundary relationship, theintergranular' corrosion resistance of "the steel is lowered.- It is believed that nickel decreases the tolerance ofthe steel for'carbonf; If nitrogen'is utilized in conjunction with nickel to form austenite, the resistance of theste'els falling on the phase. boundary line to int'ergra'nular attack can be further increased. Nitrogen-does not increase 'intergranular susceptibility to thesame extent as. carbon when the per that the in'itrogen content be held 0.15% .forthis purposejbut it should notexceed the amount that the steel-can safely fhold' in solution "during solidification as otherwise -the soundness otf 'the metal will be afiected. a

I The resistance 'of 'the steels to mer ing-in corrosion maytb'e {further increased by incorp so that the optimum composition can beselected for any given set of conditions. geveral such relationships for austenitic chromium-nickel steels iods of heating in 4 about 0.25% and preferably should be held at about 1%. The molybdenum content may be extended to 3% if greater resistance to reducing conditions is desired, but there is some sacrifice in resistance to oxidizing acid conditions when the molybdenum content exceeds about 1%.

; When fnickeli-f-isf'partially replaced 5 with nitrogen,

' it is advantageous toadd at least "-0.5% to 1% molybdenum to the steel to offset the loss of resistance to reducing conditions due to the lowering of, nickel.

The resistance of the steels to intergranular atta'ck' is'igreatlysincreased if the carbon content shhelmat a maximum of 0.04%. Low-carbon "steels are" particularly required if the steels are to be-welded; and stress-relieved during fabricartion; It has been noted that the short period of heating in the carbide precipitation temperature range during welding coupled with a heattreatment of 2 hours at 870 C. for the purpose of stress-relieving-is far more damaging toithe corrosion i'resis't'a'nce of the steel than the isothermal heattreatments normally employed'to simulate welding conditions for "test purposes. Where particularly heavy sections are "involved especiallywhere 'it is 'r'ieces'sa ry qw'e d such sections or 'otherwise heat themflt'ofa temperature within the range of 400 to 900? C}, it is advisable to keep the carbonless than 0.'0 l%.

As is well understood by those skilled in the art, it is generally'not possible to secure an exact composition, and inthe endeavor to obtain the preferred composition, the {results are likely to vary within cer'tainlimits. 'Deviationswithinflthe following ranges are permissible without fgreatly impairing the .resistance of the steel tointergranular. corrosion.

narrower range eol "Illustrative exampies sh wing the'efict "oriesub- 'jected to an a leratedeqrrpsion test c'onsisting ofimmersing' the'steelirrboiling 55% nitric an for 'five periodsf'of 4suours estahated 10 minutes at -1j075- l'hour 'at'650Clanda' iil'eaeh' series i on; manstr wn 8 ganese, nitrogen, and carbon contents to render the steel fully austenitic in the annealed condi.-. tion. In each series, the nickel content was increased beyond the amount required to form an austenitic structure, and it is clearly seen that as the total alloy content of the steel increases it becomes less resistant to intergranular attack in this test medium.

6 In Table III, are; typical examples of steels made in accordance with the invention, and the I accompanying corrosion data illustrate the ex cellent resistance to intergranular corrosion exhibited by these steels after-heating for short periods of time in the detrimental temperature range. The corrodentemployedin the tests consisted of 10% nitricacid3% hydrofluoricacid Table I Composition, Percent-Remainder Fe 9 F ggg? {f 01 N1 M11 Si O N A B 18. 37 10. 88 1. 09 0. 32 0. 024 0. 029 0. 00056 0. 00080 18. 51 11. 82 1.32 O. 51 0. 023 0. 026 0. 00058 0. 00084 17. 92 12. 64 1. 27 0. 50 0.022 '0. 027 0. 00053 0. 00069 18. O2 14. 97 1. 43 O. 47 0. 026 0. 027 0. 00055 0. 00197 20. 52 12. 71 1. 24 0. 54 0. 028 0. 039 0. 00053 0. 0950 20. 36 15. 24 1. 39 0. 47 0. 028 0. 035 0. 00039 0. 0125 20. 61 18. 32 1. 35 0. 53 0. 037 U. 034 0. 00042 0. 0350 18. 3 6. 5 1. 2 0. 4 0. 030 0. 115 0. 00088 0. 00092 18. 3 8. 1. 2 0. 4 0. 030. 0. 113 0. 00072 0. 00074 18. 25 10. 15 1. 24 0. 4 0. 030 0. 110 0. 00065 0. 00191 18. 3 6. 1. 2 0. 4 0. 033 0. 146 0. 00099 0. 00099 18. 3 8. 0 1. 2 c '0. 4 0. 033 0. 144 0. 00076 0. 00081 18. 35 10. 47 1. 20 0. 40 0. 033 0. 153 0. 00075 0. 00250 18. 31 12. 5D 1. 22 0. 46 0. 033 V 176 0. 00062 0. 00170 A=Heated minutes at 1075 C. and air-coole B=Heated 10 minutes at 1075 0., aircooledjthen heated 1 hour at 650 C. and j air-cooled.

Many tests have shown that the method of improving intergranular corrosion resistance described herein is applicable to a wide variety of compositions falling within the limits of composition set forth above. Several examples of steels containing nitrogen or molybdenum or both are given in Table II to illustrate further the inven-' tion. The steels in Table II were also tested in boiling 65% nitric acid, but, in these tests, the samples were exposed for three periods of 48 hours each after heating minutes at 1125 C.,' air-cooling, then heated 2 hours at 650 C. and air-cooling. As in Table I, the first steel in each series is in accordance with the invention- Again it is seen that as the total alloy content is raised above the amount required to form an austeriitic structure, in these cases by increasing nickel, resistance to intergranular attack decreases. It

'will be apparent that steels made by the method described herein need not be annealed after being exposed to the detrimental range for short-periods of time as in welding, to avoid subsequent intergranular attack in service.

heated to 70 C.; and the tests were conducted for 5 periods of 1 hour each. This solution is extremely corrosive and provides a sensitive and rapid test for revealing intergranular susceptibility due to carbide precipitation at the grain boundaries. Prior to heating in the detrimental range, the test samples were held 15 minutes at 1125 ,Cfan'd air cooled. The composition of the steels included in the table meets the requirements of the phase {boundary relationship for a completely'austenitic structure, and they fall within the allowable.variation limits described above. These-data clearly indicate that steelsmadeiin accordance with this invention Table I I Oompositi0n,- Pereent-e-Remainder Fe gggfizgg gg figg g Cl N1 Mn Si M0 0 N A B 18 5. 0 1. 5 0. 5 0. 057 0. 0. 00113 0. 0021 18 6. 0 1. 5 0. 5 0. 051 0. 145 0. 00101 0. 0049 18 7. U 1. 5 0. 5 U. 051 0. 144 0. 00086 0. 0054 18 6. 0 1. 5 0. 5 0. 5 0. 060 0. 166 0. 00094 0. 00108 18 8. 0 1. 5 0. 5 0. 5 0. 050 0. 158 0. 00088 0. 00180 18 10. 0 1. 5 0. 5 0. 5 0. 060 0. 139 0. 00079 0. 00825 18 7. 0 1. 5 0. 5 1. O U. 06 0. 17 I 0. 00085 0. 00098 18 9. 0 1. 5 0. 5 1. U 0. 06 0. 13 0. 00079 0. 00379 18 10. 5 1. 5 0. 5 1; 0 0. 058 0. 04 0. 00057 0. 00191. 18 15. 0 1. 5 0. 5 1. 0 0. 05G 0. 04 0. 00088 0. 00420 18 9. 0 1. 5 0- 5 2. 0- -0. 054 0; 166 0; 00076 0. 00120 is 11. 5 1. 5 0. 5, I 1 2. 0 0. 042 a 0. 159 o .,ooi41 0. 00830 A=Heated 15 minutesat 112 550. and air-cooled.. B=Heated 15 minutes at 1125 0;, air-cooled, air-cooled.

; fTableIIl- Percent Composition, Remainder Fe' and I". Corrosion Bate, Inch Penetra- 18% Or f '3 tionpermonth 6.0 1.5 0.5 0.144 0.91 i 0.37 I 0.75 0.94 r

A=Heated 15 minutes at 1125 C. and air-cooled. B==]E1Iea.ted 15 minutes at 1125 C. and air-cooled, heated 2 air-coo e iidmsat 650 0. and

O=Heated 15 minutes at 1125 C. and air-cooled, heated hour at 750 C. and

air-cooled.

D=Heated 15 minutes at 1125 C. and air-cooled, heated 2 hours at 870 C{ and air-cooled.

molybdenum; 0.02% to 0.06% carbon; 0.25% to 2.5% manganese; up to 1% silicon; and 0.1% to 0.2% nitrogen, the remainder iron,'the nickel content of such steel being substantially equal to:

(%Cr+ 1.8 (%Mo)+2.5 (%Si) my Z'JChrQniium-nickeI steel fully austenitic in the annealed condition and containing 17% to 20% chromium; 5% to 12% nickel? 0.25% to 3% molybdenum; 0.02% to 0.045% carbon; 0.75% to 1.75% manganese; 0.2% to 0.6% silicon; and

Table IV Percent Composition, Remainder Fe and Corrosion Rate, Inch Penetration 7 18% Gr 1 per month 1 Ni 1WD. Si MO 0 Y N A B; -D

0. 1. 5, 0. 5 0. 032 0. 144 0. 00094 0. 00103 0. 00090 0. 00094 0. 0 1. 5 0. 5 0. 5 0. 030 0. 16 0. 00099 0. 00145 0. 00111 0. 00092 0. 5 1. 5 0. 5 0.5 0. 042 0. 140 0; 00008 0. 00240 0. 00205 0. 00092 7. 0 1. 5 0. 5 1 1. 0 t i .0. 039 0. 1701 0. 00113 0. 00135 0. 00108 0. 00090 p 9. 0 l. 5 0. 5 2. 0 V 0 51 107 I 0. 0 0070 0. 00250 00470 0. 00150 A=Heated 15 minutes at l125 audair-coolc'd. 0 I

air-cooled.

O=Heated 15 minutes at 1l25 (Mair-cooled; then heated hour at 750 0..

D=Heated 15 minutes at ll25" (3., air-cooled, then heated 5 hours at 870 C; and

air-cooled.

In the process of proving this invention, a large number of steels have been made and tested for the purpose of establishing the limits of the invention. The data obtained permit. the conclusion that steels made=in accordance with the invention possess-superior resistance to intergranular attack. While an improvement is derived in every case, it will be apparent that not all steels embodying the invention will possess the same degree of intergranular corrosion resistance. It will be apparent, for example, that alloy constituents such as nitrogen and molybdenum, may beused with beneficial results. It will also be apparent that, While the maximum permissible carbon content varies with composition, for extremely severe conditions it is preferable to keep the carbon at about 0.04%. And finally, it will be apparent that the choice of composition will depend on the service conditions to be encountered.

This invention is in part a continuation of our application Serial No. 748,110, filed May 14, 1947, now abandoned. I

We claim: 7

1. Chromium-nickel steel fully austenitic in the annealed condition and containing 16% to 25% chromium; 5% to 25% nickel; up to 3% I file of this patent:

REFERENCES crime The following references are of UNITED STATES PATENTS Name Date Franks Sept. 23, 1041 OTHER REFERENCES Number Page 11 of paper No. 25 presented at the record in the 

1. CHROMIUM-NICKEL STEEL FULLY AUSTENITIC IN THE ANNEALED CONDITION AND CONTAINING 16% TO 25% CHROMIUM; 5% TO 25% NICKEL; UP TO 3% MOLYBEDNUM; 0.02% TO 0.06% CARBON; 0.25% TO 2.5% MANGANESE; UP TO 1% SILICON; AND 0.1% TO 0.2% NITROGEN, THE REMAINDER IRON, THE NICKEL CONTENT OF SUCH STEEL BEING SUBSTANTIALLY EQUAL TO: 